<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>输入年龄判断是否成年</title>
</head>
<body>
    
</body>
<script>
    var age = prompt('请输入年龄')
    if (age <= 17) {
        alert('你还未成年')
    } else if (age >=18) {
        alert('恭喜你成年了')
    }
</script>
</html>